The structure of a transformer is prescribed by IEC (International Electrotechnical Communication) Standards Pub. 60950 and the like. That is, these standards require that at least three insulating layers are to be formed between primary and secondary windings in a winding, that an enamel film covering a conductor of a winding is not admitted as an insulating layer, and that the thickness of an insulating layer is to be 0.4 mm or more. The standards also provide that a creeping distance between the primary and the secondary windings, which varies depending on the applied voltage, is to be 5 mm or more, and that the transformer withstands a voltage of 3,000 V applied between the primary and the secondary sides for one minute or more, and the like.
According to the standards, conventional transformers have employed such a structure as illustrated in the cross-section view shown in FIG. 2 so far. In the structure of this transformer, an enameled primary winding 4 is wound around a bobbin 2 on a ferrite core 1, in such a manner that insulating barriers 3, to secure the creeping distance, are arranged individually on the opposite sides of the peripheral surface of the bobbin. An insulating tape 5 is wound for at least three turns on the primary winding 4; additional insulating barriers 3, to secure the creeping distance, are arranged on the insulating tape, and an enameled secondary winding 6 is then wound around the insulating tape.
Recently, a transformer having a structure that includes neither the insulating barriers 3 nor the insulating tape layer 5, as shown in FIG. 1, has started to be used in place of the transformer having the structure shown in FIG. 2. The transformer shown in FIG. 1 has an advantage over that shown in FIG. 2, in that it can reduce an overall size and dispense with a winding operation for the insulating tape.
When the transformer shown in FIG. 1 is manufactured, with respect to the primary winding 4 and the secondary winding 6 to be used, at least three insulating layers 4b (6b), 4c (6c) and 4d (6d) must be formed around one or both of conductors 4a (6a) according to IEC standard.
A winding in which an insulating tape is first wound around a conductor to form a first insulating layer (an innermost layer) thereon, and is further wound to form a second insulating layer (an intermediate layer) and a third insulating layer (an outermost layer) in succession, so as to form three insulating layers that are separable from one another, is known. Further, in place of insulating tapes, it is known that fluororesins are sequentially extruded to cover the outer periphery of a conductor to entirely form three insulating layers (see, for example, JU-A-3-56112 (“JU-A” means unexamined published Japanese utility model application)).
In the above-mentioned case of winding an insulating tape, however, because winding the tape is an unavoidable operation, the efficiency of production is extremely low, and thus the cost of the electrical wire is conspicuously increased.
In the above-mentioned case of extrusion of a fluororesin, since the insulating layer is made of the fluororesin, there is an advantage of good heat resistance and high-frequency characteristic. On the other hand, because of the high cost of the resin and the property, which an external appearance is deteriorated when it is pulled at a high shearing speed, it is difficult to increase the production speed. Consequently, the cost of the electric wire becomes higher like that of the insulating tape does.
To solve such problems, a multilayer insulated wire has been put into practical use, which is obtained by extruding modified polyester resins the crystallization of each of which is controlled and a reduction in molecular weight of each of which is suppressed as the first and the second insulating layers and a polyamide resin as a third insulating layer to cover the outer periphery of a conductor (see, for example, U.S. Pat. No. 5,606,152, JP-A-6-223634 and the like (“JP-A” means unexamined published Japanese patent application)). In association with recent miniaturization of electrical and electric equipment, an influence of heat generation on the equipment has been concerned, so a multilayer insulated wire with improved heat resistance has been proposed, which is obtained by extruding a polyethersulfone resin as an inner layer and a polyamide resin as an outermost layer to cover the outer periphery of a conductor (see, for example, JP-A-10-134642).
After the winding process, the resulting transformer is installed in an instrument (machinery or tools) to form a circuit. In this process, the conductor is exposed at the tip end of the wire drawn out of the transformer and soldered. As electrical and electric instrument has been made more compact, however, there has been a demand for multilayer insulated wires whose coating layers cause no cracking even when part of the covered conductor is drawn out of a transformer, subjected to working such as bending, and then soldered, and in which working such as bending is favorably performed on the covered conductor after soldering.